Move libc++ configs and flags out of //build/config/sanitizers

build/config/c++/BUILD.gn

+# Copyright 2017 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("//build/config/c++/c++.gni")
+
+config("c++flags") {
+  if (use_custom_libcxx) {
+    prefix = "//buildtools/third_party"
+    include = "trunk/include"
+    cflags_cc = [
+      "-nostdinc++",
+      "-isystem" + rebase_path("$prefix/libc++/$include", root_build_dir),
+      "-isystem" + rebase_path("$prefix/libc++abi/$include", root_build_dir),
+    ]
+  }
+}